Matching Network Design for Wideband Power Amplifier Cascaded with Harmonic Suppression Filter

Abstract

When a wideband power amplifier (PA) is cascaded with harmonic suppression filter, transmission zeros of the filter influence the harmonic impedances seen by the transistor. As a result, output power of the module consisting of PA cascaded with filter is not the power of PA reduced by insertion loss of filter (additivity is lost). This anomaly can be corrected by incorporating harmonic termination mechanism in conventional constant Q matching network. Modified matching network designed by this method retains its harmonic impedance when cascaded with harmonic suppression filter. Design has been validated by comparing performance of PA designed with constant Q matching network and modified matching network. PA with conventional matching network shows 1.5dB drop in power at centre of frequency band after cascading with harmonic suppression filter of 0.6dB insertion loss whereas, PA with the modified matching network shows 0.6dB drop in power at higher edge of band (and lesser at lower edge) after cascading with harmonic suppression filter. Besides restoring the desired additivity when cascaded with filter, this matching network also acts as an integrated harmonic suppression filter. The designed amplifier and filter have been implemented in 73x38 sq.mm and 49x37 sq.mm planar microstrip technology respectively and hence can be seamlessly integrated in compact man-pack radios. Fabricated module consisting of PA and filter delivers 50W power and achieves harmonic power level as low as 80dBc.